A myriad of displacement sensors are used in industrial, commercial and residential applications. These include, for example, inclinometers and accelerometers. The accuracy and effectiveness of such sensors are generally limited by their inherent hysteresis.
Displacement sensors generally comprise a base for attaching the sensor to other objects and an enclosure that is attached to the base and contains a sensing unit. The base may be of any convenient shape or in any convenient orientation. The sensing unit typically comprises two kinds of elements. One kind of element in the sensing unit is typically fixed relative to the base, while another is configured such that it can move relative to the fixed elements. The relative motion between the fixed and moving elements in the sensing unit, which is typically a function of the displacement of the base, is measured and used to determine the displacement of the sensor base.
In one class of displacement sensors, the sensing unit comprises a liquid element contained in a vessel. When the sensor is displaced, the liquid element moves relative to the vessel. The position of the liquid with respect to one or more vessel wall segments is detected and used to determine displacement, inclination or acceleration of the sensor base. U.S. Pat. No. 4,624,140, the contents of which are hereby incorporated herein by reference in their entirety, describes an inclinometer with a sensing unit comprising a vessel that is partially filled with a conductive liquid. The sensing unit further comprises conductive wall segments that are held in a fixed position relative to the inclinometer base and at least one of which is coated with a thin dielectric coating. In use, when the inclination of the base of such an inclinometer is varied, the conductive liquid covers a variable portion of at least one dielectric coated wall segment. The capacitance between the conductive liquid and the at least one dielectric coated wall segment, which varies as a function of the inclination of the base of the device, is used as a measure of displacement of the sensor. The liquid element may be any conductive flowable material that conforms to the shape of the vessel in which it is placed during use. It may contain multiple liquids, solids and other dissolved or undissolved materials.
An alternate capacitive sensor, which uses a low conductivity liquid as the dielectric of a variable capacitor, is disclosed in U.S. Pat. No. 3,906,471, the contents of which are incorporated herein by reference in their entirety. U.S. Pat. Nos. 4,912,662 and 5,083,383, the contents of which are incorporated herein by reference in their entirety, also describe other configurations of inclinometers using a liquid element in the sensing unit.
Generally, the accuracy of inclinometers and other displacement sensor technologies, with and without liquid sensing elements, are limited by hysteresis. Hysteresis in such sensors is at least partially caused by stickiness or adhesion between the elements of the sensing unit that are fixed with respect to the base of the sensor and those elements that are intended to move relative to the base when the base is displaced. In liquid filled sensors, stickiness or adhesion between the liquid element and solid surfaces that it contacts causes or contributes to the hysteresis of the sensor.